Rosa Rugani, University of Padova
She's Got a Number Line
Think about a series of numbers, 1 through 10. Chances are,
you imagined the numerals in a line, in increasing order from left to right.
That's the effect of your mental number line, the mental representation of
numbers that research has shown basically all humans have—and, it seems, some
non-humans, too. To put it another way, human adults tend to imagine numbers as
being ordered in space, going a certain direction. It's a phenomenon that seems
to be nearly universal. Pretty cool.
The directions of people's mental number lines are affected by
the directionsin which they read their primary languages. Do animals and
babies have mental number lines, too? That's iffier to answer, although
psychologists have done studies that answer "yes" to both. Now
there's a new study that suggests three-day-old chicks have mental number
lines. Their lines go left to right, if you're curious.
To test chicks' mental number lines, psychologists in Italy
taught the chicks that they could find food behind a card with dots on it. To
start, they showed the chicks only single cards, each with five dots. Then the
psychologists began putting chicks in little setups where the birds could
choose between two cards with some number of dots other than five. Both cards
would have the same number of dots. Sometimes the cards would have two dots,
while other times, they would have eight. The psychologists found that the
chicks were more likely to walk to the left card, if the cards showed two dots.
The birds were more likely to walk right if the cards showed eight dots. Voilà !
The chicks were associating smaller numbers with the left side, and larger
numbers with the right!
The effect worked even when the psychologists used a
baseline training number other than five. Chicks trained on 20-dot cards tended
to walk left when shown two cards with eight dots on them each.
It's hard to really read the minds of chicks (ba-dum
tshhh!), so it's possible the chicks were responding to something other than a
mental number line. The same is true for other mental-number-line experiments
researchers have conducted on nutcracker
birds, macaques, and chimpanzees.
If animals do have mental number lines, however, that
suggests that the phenomenon arose early in the evolution of vertebrates. And why
not? Understanding magnitude is clearly important to survival and there's
strong evidence many animal species count. Maybe they do it just like we do,
along a line in space.
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